1. Field of the Invention
This invention pertains in general to the field of delivery of volatile liquid anesthetic agents by means of an anesthetic breathing apparatus. More particularly the invention relates to a control method and system in an anesthetic breathing apparatus when multiple anesthetic agents are present in a breathing circuit of the anesthetic breathing apparatus to which a patient is fluidly connected via its airways.
2. Description of the Prior Art
During anesthesia, the patient is usually supplied with a gas mixture containing nitrous oxide, oxygen and an anesthetic agent. Generally, one of the volatile anesthetic agents desflurane, enflurane, isoflurane, halothane or sevoflurane is the anesthetic employed. These are all liquid at normal room temperature.
A liquid volatile anesthetic agent administered to the patient via its airways is therefore vaporized in an anesthetic vaporizer that is fluidly connected to a breathing circuit of an anesthetic breathing apparatus, such as an anesthesia machine. The vaporized anesthetic agent is conveyed to the breathing circuit with a flow of fresh gas that is suitably controlled. The patient in turn is fluidly connected to the breathing circuit via a suitable tubing.
The breathing circuit of the anesthetic breathing apparatus is usually, of economical reasons, operated such that the patient re-breathes exhalation gases after CO2 is removed in a CO2 absorber, and the O2 level in the inhalation gases is adjusted by replenishing the latter in the breathing circuit. The above mentioned fresh gas flow is kept much lower than the patient gas flow from and to the breathing circuit. Thus it is ensured that only a minimum of liquid anesthetic is exhausted from the anesthetic breathing apparatus, e.g. to a gas evacuation system.
A mixing of different anesthetic agents is attempted to be avoided, as pharmacological summation effects thereof are still not fully understood and may vary from one patient to another. For instance in U.S. Pat. No. 6,289,891 a safety system is disclosed that avoids mixing of different anesthetic agents in an anesthetic vaporizer of an anesthetic breathing apparatus.
However, under certain operating conditions of the anesthetic breathing apparatus, it might still occur that more than one anesthetic agent is present in the breathing circuit. For instance, an anesthetic breathing apparatus may comprise a number of anesthetic vaporizers. Multiple anesthetic agents might exist for instance in a transition phase when switching between two different anesthetic agents delivered from two anesthetic vaporizers of the anesthetic breathing apparatus, each delivering a different anesthetic agent. For instance a first anesthetic agent may be provided from a first anesthetic vaporizer during an induction phase of anesthesia. Subsequently, in order to maintain the level of anesthesia established by the first anesthetic agent, a second anesthetic agent may be delivered to the breathing circuit and therefrom to the patient from a second anesthetic vaporizer. This might also be desired if a patient shows symptoms of not tolerating a certain anesthetic agent, e.g. due to allergic reactions. In cases like this, multiple anesthetic agents may be present in the breathing circuit, at least for a certain time, for instance when operating the anesthetic breathing apparatus with low fresh gas flows into the breathing circuit.
Usually, in order to avoid that multiple anesthetic agents are present in the breathing circuit, the latter may be flushed for removing all anesthetic agents as fast as possible therefrom. Then a new anesthetic agent will be provided with a rather high gas flow to replenish the breathing circuit with the new anesthetic agent. Thus, the patient will not significantly loose depth of anesthesia during this flush and replenish procedure, as sufficient anesthetic agent is dissolved in the patient's blood serving as a depot until new anesthetic agent is provided to the patient. On the other hand, the breathing circuit, that has been flushed and replenished with the new anesthetic agent, will receive a certain amount of the accumulated anesthetic agent from the patient with exhalation gases comprising anesthetic agent dissolved from the blood of the patient. This includes the previous anesthetic agent, whereby the concentration thereof in the breathing circuit will increase again for a certain time after the flush and replenish procedure, until the previous anesthetic agent is washed out from the breathing circuit or eventually consumed by the patient. Hence, the change from one anesthetic agent to another anesthetic agent supplied to a patient may be time consuming and involve mixtures of multiple anesthetic agents in the breathing circuit that are not known or not desired. One way to accelerate removal of the previous anesthetic agent would be to regularly repeat the flush and replenish procedure. However, the flush and replenish procedure is not desired from an economical point of view as a large amount of anesthetic agent is wasted from the anesthetic breathing apparatus to the exhaust thereof during such a procedure.
In addition, if for instance due to the human factor safety locks to activate more than one vaporizer at a time are overridden, it may also occur that a mixture of several anesthetic agents are delivered to the patient.
Thus, there is a need for an improved system and/or method for providing or regulating a plurality of anesthetic agents in a breathing circuit, e.g. without impairing patient safety.
Hence, an improved anesthetic breathing apparatus would be advantageous, and in particular an anesthetic breathing apparatus allowing for increased flexibility, and/or cost-effectiveness, and/or patient safety, with regard to multiple anesthetic agents in a breathing circuit thereof would be advantageous.